Chemists
at Italy's University
of Bologna , UCLA and the California
NanoSystems Institute (CNSI) have designed and
constructed a molecular motor of nanometer size that
does not consume fuels; their nano motor is powered
only by sunlight. The research, federally funded
by the National Science Foundation, will be published
Jan. 31 in Proceedings of the National Academy of
Sciences (PNAS).

The nano motor can work continuously without any external interference, and operates
without consuming or generating chemical fuels or waste, said Fraser Stoddart,
UCLA's Fred Kavli Professor of NanoSystems Sciences and CNSI Director.

“We design and make sunlight-powered nano motors
and then ‘test drive' them much as an engineer would
a new motor car,” Stoddart said. “It is as if we
had managed to get a solar powered motor car onto
the road and running.”

Precisely how light-powered nano motors will be used in the future is not yet
clear, Stoddart said, but he listed a number of possible areas for applications:
nanoelectronics, molecular computers, and nano valves that perhaps could be
used for the delivery of anti-cancer drugs and other medications.

“The achievement reported in PNAS is the culmination of a research effort lasting
a quarter of a century and involving hundreds of students and millions of dollars,” Stoddart
said.

What is the nano motor's composition, and how does it work?

The nano motor is a multi-component molecular-scale system called a rotaxane,
a mechanically interlocked molecule consisting of one or more rings trapped
on a rod by bulky stoppers at either end, in a manner reminiscent of an abacus.
The system is built up from two separate molecular components: a dumbbell-shaped
one, which is more than six nanometers long, and a ring component of a diameter
of approximately 1.3 nanometers. The ring component is trapped on the rod portion
of the dumbbell by two very bulky stoppers attached to the ends of the rod
so that, although the ring can move along the rod, it cannot go over the stoppers
at the ends. The rod portion of the dumbbell contains two “stations” that can
be called A and B.

“It is the attractiveness between the ring and stations A and B that assists
us in making the molecules in the first place,” Stoddart said. “The attractiveness
for these two stations (A and B) lives on in the two-state or bistable rotaxane
after it has been made. The final requirement in the design of the nano motor
is that the ring prefers, in the starting state of the molecule, to surround
one of the two stations, let us say A. In order to induce the ring to move from
A to B we have to make A temporarily a less desirable station such that the ring
will spontaneously migrate to station B.

“The linear nano motor works as follows. The absorption of sunlight by one of
the two stoppers, a light-harvesting one, causes the transfer of one electron
to station A, which is deactivated as far as wanting the ring to encircle it.
As a consequence, the ring moves to its second port of call, station B. Station
A is subsequently reactivated by the return of the transferred electron to the
light-harvesting stopper, and the ring moves back to this station.

“The system operates according to a four-stroke cycle which is reminiscent of
an internal combination engine in a motor car: (1) light excitation and subsequent
transfers of an electron (“combustion”), (2) displacement of the ring along the
rod from A to B (“piston displacement”), (3) removal of the electron received
by station A (“exhaust removal”), and (4) relocation of the piston. The motions
executed by the nano motor are quite rapid: a full cycle is carried out in less
than one thousandth of a second, which means that the motor can operate at a
frequency of 1000 Hertz – a speed that is equivalent, using the car engine analogy,
to 60,000 RPM.”

Where does the research go from here?

“The research will have to go in the direction of taking the nano motors and
putting them on surfaces and into membranes,” Stoddart said. “The science and
engineering to achieve these objectives is daunting and tough, but the rewards
are correspondingly great and satisfying. The kind of nanotechnology that will
emerge from these nano motors still requires a lot of fundamental work. The nano
motors are extremely sophisticated in their design.”

The nano motor was designed, assembled, and run by the research groups at UCLA
and the University of Bologna working closely together.

The international research team included Vincenzo Balzani, Alberto Credi and
Margherita Venturi from the University of Bologna's Photochemistry and Supramolecular
Chemistry Group.

The research was also funded by Italy's Ministry of Education, University and
Research, and by the European Union.